The present invention relates to an apparatus for treating a ceramic object in an alkali hydroxide melt, the apparatus being of the type which includes a heatable crucible for containing an alkali hydroxide melt in which ceramic objects are immersed, a heatable housing which is connected to the crucible in a substantially gas-tight manner to provide a preheating chamber above the crucible, a holder for moving the ceramic objects through the preheating chamber to the melt, and a nozzle arrangement which feeds a fluid mixture such as vapor and gas to the surface of the melt. The invention relates, in particular, to an apparatus for treating ceramic plates which are then metallized and processed into electrical circuits.
Ceramic objects are metallized to change, improve, or supplement the functional characteristics of the ceramic objects, such as electrical or thermal conductivity, resistance to corrosion, resistance to wear, or decorative characteristics. For such metallizations, the adhesion of the metal coating on the ceramic material is of particular significance. Very generally speaking, the adhesion is produced either by a relatively weak interaction between the material of the layer and the substrate (so-called Van der Waals forces), by chemical bonds, by metallic anchoring, or by a combination of these contributory factors. The percentages of these individual contributory factors depend primarily on the type of pretreatment received by the ceramic substrates. For example, the degree of adhesion provided by mechanical anchoring can be increased by a pretreatment which roughens the ceramic substrates.
Generally, the ceramic surfaces must be freed of their so-called glass-like firing skin, which makes most types of ceramic materials chemically inert. A suitable process must be employed to do this in a reproducible manner. The ceramic surfaces may be cleaned and activated in a chemical removal process so that chemical bonds can be developed with the surfaces of the ceramic crystallites that have been exposed. However, this removal process must neither excessively roughen the surfaces nor loosen the structure of the material since otherwise mechanical attachments would break out too easily. A chemical etching process which primarily removes the glass-like firing skin is therefore most suitable for this purpose. A number of etching agents have been proposed in the literature (see R. Bock: Aufschlussmethoden der anorganischen und organischen Chemie [Decomposition Methods Used in Organic and Inorganic Chemistry], published by Verlag Chemie, Weinheim/Bergstrasse, 1972) for treatment of aluminum oxide, which is the ceramic substrate material most commonly used in the electronics art. Examples of these etching agents are phosphoric acid, sulfuric acid, nitric acid and hydrofluoric acid or ammonium hydrogen fluoride, and sodium hydroxide solution or a sodium hydroxide melt. However, the acids have been found to be generally lacking in efficiency while the alkalis attack the ceramic surfaces only at a very high temperature, and then too strongly or irregularly. If the method of immersion pretreatment with a sodium hydroxide solution is employed, the layer must first be dried, where then the drawback of the danger of carbonate formation exists, and then fired. The firing must occur at a very high temperature (approximately 500.degree. C.), with the small quantities of alkali hydroxide being distributed unevenly and going quickly through their reaction with the substrate so that no further etching can take place.
In the method of immersion pretreatment in a melt of an alkali hydroxide, no significantly stronger etching action can take place with sodium hydroxide even above the melting point of, for example, 318.degree. C. An additional, grave drawback of this method is the fact that these extremely aggressive melts are difficult to handle, since they strongly attack practically all conventional instrument materials and since they form carbonates very easily by taking carbon dioxide from the ambient air, which adversely affects the etching conditions.